1. Field of the Invention
The present invention relates to a waveguide optical modulator for modulating the power of light output from a light source.
2. Description of the Related Art
Heretofore, optical transmitters for use in optical fiber communication systems have employed a direct modulation process for modulating a current flowing through a laser diode with a data signal. However, it is difficult for the direct modulation process to transmit signals over long distances because an output light signal suffers more chirping as the transmission rate is higher due to the wavelength dispersion occurring in the optical fiber.
In view of the above difficulty, there has been put to practical use an optical transmitter wherein an external modulator that is insusceptible to chirping in principle acts on CW (continuous-wave) light. One known external modulator of this type is a waveguide optical modulator having optical waveguides of a predetermined pattern formed on a waveguide substrate to provide a Mach-Zehnder interferometer and electrodes for applying an electric field to the Mach-Zehnder interferometer.
The waveguide optical modulator has a first Y-branch optical waveguide providing an input port, a second Y-branch optical waveguide providing an output port, first and second optical waveguides interconnecting the first and second Y-branch optical waveguides, first and second signal electrodes disposed respectively on the first and second optical waveguides, and a ground electrode having a potential difference with the first and second signal electrodes and operable in coaction with the first and second signal electrodes for applying an electric field to the first and second optical waveguides.
The power of a light beam is divided into two equal levels by the first Y-branch optical waveguide. The light beams are propagated through the first and second optical waveguides. When the light beams are combined with each other in phase (the phase difference is 2nπ (n is an integer)) by the second Y-branch optical waveguide, the light output of the waveguide optical modulator is turned on. When the light beams are combined with each other out of phase (the phase difference is (2n+1)π) by the second Y-branch optical waveguide, the light output of the waveguide optical modulator is turned off. Therefore, the waveguide optical modulator can modulate the power of the light beam with less chirping when the voltages applied to the electrodes are changed by an input signal.
FIGS. 1A and 1B of the accompanying drawings are a plan view and a cross-sectional view taken along line b-b of a conventional waveguide optical modulator having a Z-cut LiNbO3 substrate. The waveguide optical modulator includes a Y-branch optical waveguide 4 providing an input port 2, a Y-branch optical waveguide 8 providing an output port 6, and optical waveguides 10 and 12 interconnecting the Y-branch optical waveguides 4 and 8, the waveguides being formed on a waveguide substrate 14. A manufacturing process, and details of the structure and operating principles of the waveguide optical modulator will be described below.
With Ti patterned to the same shape as the waveguides 4, 8, 10 and 12 on the waveguide substrate 14 made of LiNbO3, the assembly is heated at 1050° C. for 7 to 10 hours to form the waveguides 4, 8, 10 and 12 having a high refractive index due to thermal diffusion. If the LiNbO3 substrate is of a Z cut, then since a strong electric field is required in a Z direction along the thickness of the waveguide optical modulator, a signal electrode 16 and a ground electrode 18 are patterned respectively directly above the optical waveguides 10 and 12. To prevent light propagated through the optical waveguides 10 and 12 from being absorbed by the electrodes 16 and 18, a transparent buffer layer 20 of SiO2 is deposited to a thickness ranging from 0.2 to 1.0 μm on the waveguides 4, 8, 10 and 12. The electrodes 16 and 18 are formed of Au to a thickness ranging from 3 to 20 μm on transparent buffer layer 20.
To energize the optical modulator, as shown in FIG. 1A, the signal electrode 10 and the ground electrode 12 has their terminal ends connected to each other by a resistor R, providing a progressive-wave electrode, and a microwave input signal V in the range from several GHz to 100 GHz is applied to an end of the progressive-wave electrode. An electric field 22 is now generated between the electrodes 16 and 18. Since the refractive index of the optical waveguides 16 and 18 changes between +Δn and −Δn, input light having a wavelength λ as it is modulated to an on- and off-state by the input signal V is output from the output port 6.
With the optical modulator having the progressive-wave electrode, the waveguide substrate may operate as a microwave resonance chamber which causes a particular microwave frequency range to resonate, resulting in poor frequency vs. response characteristics. Specifically, a dip may be produced in the frequency vs. response characteristics which represent the relationship between transmission losses from the input to output ports of the optical modulator and frequencies, as shown in FIG. 2 of the accompanying drawings.
A single-drive optical modulator having one set of signal and ground electrodes as shown in FIGS. 1A and 1B is capable of suppressing the dip in the frequency vs. response characteristics by reducing the width of the ground electrode to provide an electrode-free region on the chip. However, a dual-drive optical modulator having two sets of signal electrodes, as described later on, cannot incorporate the solution of the single-drive optical modulator, and is unable to suppress the dip in the frequency vs. response characteristics.